1. Field of the Invention
The present invention relates to a light emitting diode used for illuminating a variety of electronic devices and also in recent years for illuminating Panel meters of automobiles, and in particular, relates to a light emitting diode provided with a reflection cup to give an illuminating light a desired directivity.
2. Description of Related Art
An example of light emitting diode of this kind is disclosed in Japanese Patent Disclosure No. 2002-324917. This light emitting diode, as shown in FIG. 1, comprises a base substrate 2 surface-mounted on a mother board 1, a light emitting element 3 put on an upper surface of the base substrate 2, a reflection cup 4 arranged to surround the light emitting element 3, a resin sealant 5 sealing the light emitting element 3 and the reflection cup 4, and a dome-shaped light collecting member 6 placed on the resin sealant 5.
In such a light emitting diode, light emitted from the light emitting element 3 is reflected by an inner circumferential surface of the reflection cup 4 to be given an upward directivity and then passes through a lens portion 7 of the light collecting member 6 to have a stronger directivity.
In recent years, there is a growing demand for a light emitting diode with a so-called narrow directivity, which focuses light emitted from a light emitting element and illuminates the focused light on a point. To realize such a narrow directivity using only a reflection cup requires a roughness height rating of the reflection cup surface to be less than or equal to one-half the wavelength λ of light emitted from the light emitting element in order to control the light directivity.
Roughness height rating is defined as a measure of the surface finish or the arithmetical average from a true flat surface.
However, with the conventional light emitting diode described above, since many of the reflection cups are resin-molded products, there is a limitation on the roughness height rating of their cup surfaces. Even if the surface of the reflection cup is deposited or plated with a metal film, the surface roughness height rating which is less than or equal to one-half the wavelength λ of light emitted from the light emitting element, required for the control of light directivity is difficult to achieve. Thus, the conventional reflection cup produces scattered rays of light, making it impossible to realize a desired narrow directivity with only the conventional reflection cup. It has been therefore necessary to provide an additional member such as the light collecting member 6, rendering the device that much costly. Other problems of the conventional device include a nonuniform color of emitted light due to chromatic aberration and an increased size of the light emitting diode.